1. Field of the Invention
The present invention relates to a wireless charging system and its control method and more particularly to a high efficiency wireless charging system and its control method.
2. Description of Related Art
Wireless charging is also known as contactless charging. A wireless charger transmits power to a charged device by inductive coupling, wherein a battery of the charged device is wirelessly charged by the transmitted power. Therefore, cable connections are not required during wireless charging, such that, wireless charging is safer, more durable and more convenient than conventional cable charging.
When a transmission distance between the wireless charger and the charged device exceeds an effective transmission distance of normal inductive coupling, the wireless charger has to transmit the power by resonant inductive coupling to increase the effective transmission distance of wireless charging. The wireless charger and the charged device have to be operated in same operating frequency range.
With reference to FIG. 10, a vertical axis indicates transmission efficiency of a wireless charger, and a horizontal axis indicates operating frequency of the wireless charger. One can obtain that the wireless charger has two resonant frequency points ω1, ω2, wherein the two resonant frequency points ω1, ω2 are respectively determined based on capacitive elements and inductive elements of a transmitting circuit of the wireless charger. The two resonant frequency points ω1, ω2 can be respectively calculated according to two formulas:
            ω      1        =                  1                  C          ⁡                      (                          L              +                              L                m                                      )                                                  ω        2            =                        1                      C            ⁡                          (                              L                -                                  L                  m                                            )                                            ,  wherein Lm is coupled inductance.
When any one of the two resonant frequency points lies in the operating frequency range of the wireless charger, the resonant frequency point which lies in the operating frequency range of the wireless charger is defined as an optimal frequency point.
However, the optimum frequency point may be beyond the operating frequency range when the transmission distance between the wireless charger and the charged device varies, and decreases the transmission efficiency of the wireless charger.
With reference to FIG. 11, a vertical axis indicates the sending efficiency of the wireless charger, and a horizontal axis indicates the transmission distance between the wireless charger and the charged device. One can obtain that when the transmission distance between the wireless charger and the charged device is 18 cm, the wireless charger has optimal transmission efficiency. When the transmission distance between the wireless charger and the charged device changes from 18 cm, no matter the transmission distance is extended or shortened, the transmission efficiency of the wireless charger is reduced.
Therefore, how to keep the resonant frequency points in the operating frequency range is an important object in wireless charging, wherein impedance matching is usually used for keeping resonant frequency points of a wireless charger in the operating frequency range of the wireless charger. There are two conventional methods for impedance matching:
1. adding an impedance matching circuit to the wireless charger.
2. adjusting parameters of a power amplifier of the wireless charger.
With reference to FIG. 12, an impedance matching circuit 80 is added to a coupling antenna 70 of a wireless charger. The impedance matching circuit 80 has two capacitors CS, CP and an inductor LS, wherein capacitance values of the two capacitors CS, CP and an inductance of the inductor LS are adjustable. By the two above-mentioned formulas, one can obtain that resonant frequency points of the wireless charger change with a variation of the capacitance values of the two capacitors CS, CP and/or a variation of the inductance of the inductor LS. Therefore, the resonant frequency points can be adjusted to lie in the operating frequency range of the wireless charger by adjusting the capacitance values of the two capacitors CS, CP and/or the inductance of the inductor LS.
However, adding an impedance matching circuit to the wireless charger has disadvantages of a low matching accuracy, a low matching speed, and even lowering a transmission efficiency of a wireless charger.
Besides, adjusting the parameters of the power amplifier of the wireless charger also has disadvantages of a low matching accuracy and having a complicated calculation.
In conclusion, the conventional methods for keeping optimal frequency points in an operating frequency range of a wireless charger by impedance matching have multiple disadvantages of a low matching accuracy, a low matching speed, and even lowering the transmission efficiency of the wireless charger. Therefore, methods for keeping optimal frequency point have to be improved.